This invention relates to attaching leads for external electrical connection to metallized substrates, and in particular to a method of solder bonding the leads to the substrates.
In the field of film circuits, components are formed on an insulating substrate which typically comprises ceramic. These components can be in the form of thin and/or thick film devices with associated conductors and bonding pads for connection to outside circuitry. The circuit can further include integrated circuit chips bonded to pads on the substrate to form a hybrid integrated circuit.
In the fabrication of such circuits, an important step is bonding leads to the bonding pads on the substrate to permit external connection. The leads are typically in the form of lead frames which provide a unitary structure including metal fingers addressing one edge (Single In-Line Package) or two edges (Dual In-Line Package) of several substrates. The ends of the leads typically include jaws for clipping onto the substrate pads. However, soldering of the leads is typically required to insure a firm bond between the leads and pads.
A standard method of applying the solder is to deposit a solder paste to the pads by standard screen printing prior to attachment of the leads. After lead attachment, the solder is reflowed to provide the bond. Although generally adequate, this method presents yield problems since the solder paste can chip when the leads are attached, resulting in insufficient solder to provide a good bond, and residual solder balls tend to form on the substrate after solder reflow, producing shorts on certain portions of the circuit. Other methods commonly used, such as providing a solder preform or cladding on the leads, also have a tendency to result in variations in solder thickness at the connection between the leads and bonding pads. A further method, where the substrate and leads are immersed into molten solder, would generally produce variations in the solder thickness on the lead frames. This, in turn, interferes with subsequent forming operations for the leads.
Consequently, it is a primary object of the invention to provide a method of solder bonding external leads to film circuits which results in good control of solder thickness and high yield.